In recent years, attention has been paid to thin film photoelectric conversion devices, which have little problem about resources, and developments thereof have been actively made in order to reduce the costs of photoelectric conversion devices compatible with high efficiency thereof. A thin film silicon solar cell, which is one of the thin film photoelectric conversion devices, can be formed on a large-area glass substrate or stainless steel substrate at low temperature; thus, it can be expected that costs thereof are reduced. In order to improve the conversion efficiency of the thin film silicon solar cell, attempts for increasing the optical path of light incident into its photoelectric conversion layer have been hitherto developed or realized as a method for increasing the absorption quantity of the sunlight.
In the case of, for example, a thin film silicon solar cell wherein a glass substrate is used, the scattering of incident light is increased by a method of depositing a tin oxide (SnO2) film as a transparent electrode by thermal CVD, thereby creating a texture (microscopic structure) that may be of various types; a method of etching the surface of the transparent electrode, thereby forming a texture; or a method of depositing a layer having an irregular structure between the transparent electrode and the glass substrate.
From the viewpoint of aiming for compatibility between reducing the costs of thin film photoelectric conversion devices and high efficiency thereof, multi-junction type photoelectric conversion devices have been developed, in which two or more photoelectric conversion units having absorbable wavelength ranges different from each other are stacked, in order to make good use of a main wavelength range (400 to 1200 nm) of sunlight. For example, a double-junction type thin film silicon solar cell has been put into practical use, in which an amorphous silicon photoelectric conversion unit using light having wavelengths up to about 800 nm and a crystalline silicon photoelectric conversion unit using longer wavelengths than those used by the amorphous silicon are stacked.
For a substrate with a transparent conductive film used for such a multi-junction type photoelectric conversion device, a technique of scattering light rays having a long wavelength range by a texture having irregularities relatively large in size, and scattering light rays having a shorter wavelength range by a texture having irregularities relatively small in size is suggested, thereby widening the range of wavelengths of light to be scattered to improve photoelectric conversion properties of the device.
According to, for example, Patent Document 1, in a method for forming such textures, fine structures are formed in a surface of a glass by mechanical polishing. However, the method has drawbacks that a surface having fine irregularities is not easily obtained, and the area of the surface is not easily made large, either.
According to Patent Document 2, an irregular structure is formed by etching a surface of a transparent electrode. However, only a crater-shaped-irregular structure can be formed. Thus, it cannot be stated that this method is favorable for forming a pyramidal or inverse-pyramidal irregular structure, which is advantageous for light scattering.
According to Patent Document 3, a pyramidal or inverse-pyramidal irregular structure is formed in a glass substrate. However, high temperature is required for softening the glass in order to form the irregular structure in the glass. Moreover, the size of the recesses and projections is limited so that satisfactory effects are not necessarily obtained for light scattering of wavelengths in a wide range.
According to Patent Document 4, a pyramidal or inverse-pyramidal irregular structure is formed by applying a sol-form transparent electrode onto a substrate. However, in the same manner as described above, the size of the each irregular structure is limited. Moreover, because of the use of the sol-form electrode, a high-quality film is not easily formed.
In the meantime, a method of forming irregularity-species different from each other size in a transparent electrode is suggested, thereby widening the range of wavelengths of light to be scattered. For example, Patent Document 5 proposes a method of forming a texture with small irregularity-size in the surface of the transparent conductive film, in which a discontinuous first layer having large irregularity-size, which is made of a first oxide material with a low electroconductivity, is deposited on a flat substrate, and then a continuous layer made of a second oxide material is deposited thereon by normal-pressure CVD. More specifically, according to Patent Document 5, an amorphous SiO2 layer, as an underlying layer, is deposited on a discontinuous texture composed of a first oxide and a substrate, to give a thickness of 2 to 40 nm, and then an SnO2 film doped with fluorine is deposited thereon by normal pressure CVD, to form a second oxide layer.
As described above, according to Patent Document 5, depositing a different oxide layer made of amorphous SiO2 or the like is necessary to form the small size texture in the transparent conductive film surface. Thus, the step of producing the transparent conductive film is complicated. As shown in a scanning electron microscope (SEM) photograph in FIG. 14, the shape of the irregularities in the texture large in irregularity-size in the transparent conductive film of Patent Document 5 is not pyramidal. Further the texture formed of the first oxide material is discontinuous, and therefore, the surface of the transparent conductive film has many flat areas. For these reasons, it is not necessarily stated that a structure advantageous for a light scattering layer is formed. Furthermore, according to Patent Document 5, the irregularity-size of the texture in the surface of the transparent conductive film is large as shown in FIG. 14. Thus, in a semiconductor layer, such as a silicon layer, which is deposited on the transparent conductive film, defects are easily generated from the recesses as nucleation points. Thus, there remains a problem that, in particular, the open circuit voltage (Voc) is reduced.